You might tune your TCP/IP stack if you haven't done so. Google for tuning ftp to find guidance if this is unfamiliar.

Update: I found http://www.csm.ornl.gov/~dunigan/netperf/bulk.html particularly interesting, though it may not be applicable to your situation.

Update: And don't forget your disk I/O, system bus, etc. The network may not be your bottleneck.